Heretofore, in a thin film device such as a gate-insulated field effect thin-film-transistor (TFT) or the like, a silicon oxide film with good characteristics, which is obtained by forming a crystalline silicon followed by heating and oxidizing its surface at high temperatures falling within a range of from 900.degree. to 1100.degree. C., has been used as a gate-insulating film.
The oxide film formed by such thermal oxidation is essentially characterized in that its interfacial level density is extremely low and that it may be formed on the surface of a crystalline silicon at a uniform thickness. Accordingly, the former brings about good on/off characteristics and long-term reliability on bias/temperature; while the latter reduces the short circuit between a gate electrode and a semiconductor area (active layer) at the edges in an island semiconductor region to thereby improve the production yield of semiconductor devices.
To use such a thermal oxide film in producing semiconductor devices, however, a material which is resistant to high temperatures must be selected as the material for the substrate. In this respect, since inexpensive glass materials (for example, alkali-free glass such as Corning 7059, etc.) cannot be used, the production costs are disadvantageously high especially when large-area substrates are used. Recently, a technical means for forming TFT on an alkali-free substrate is being developed, in which, however, a thermal oxide film cannot be used but a gate-insulating film shall be formed by sputtering or by physical or chemical vapor deposition (CVD) such as plasma CVD or reduced pressure CVD.
However, it was inevitable that the characteristics of the silicon oxide film formed by such means were inferior to those of the thermal oxide film. Namely, the interfacial level density of the former is generally large and, additionally, the former was always accompanied by the dangers of alkali ions such as sodium ions or the like invading the film being formed. In addition, since the step coverage of the silicon oxide film is not so good, the film frequently caused the short circuit between the gate electrode and the active layer at the edges of the island semiconductor region. For these reasons, it was extremely difficult to obtain semiconductor devices of the kind satisfying all the characteristics, the reliability and the production yield by the prior art technology.